Performance Evaluation of Wi Max Systems Metehan Dikmen

Performance Evaluation of Wi. Max Systems Metehan Dikmen and Mehmet Şafak Hacettepe University Dept. of Electrical and Electronics Engineering 06532 Beytepe, Ankara, Turkey

Outline • • Motivation Physical layer IEEE 802. 16 extensions Wi. Max System Model – – – Randomization FEC Modulation Interleaving OFDM • Simulation – AWGN channel – Rayleigh fading channel • Results and Conclusions

Motivation • Wireless. MAN provides network access to buildings through exterior antennas communicating with BSs • Offers an alternative to cabled access networks, – e. g. , fiber optic links, coaxial systems using cable modems, and digital subscriber line (DSL) links • Have the capacity to provide high-rate network connections over broad geographic areas without costly infrastructure required in deploying

Motivation • Supports different transport technologies, including IPv 4, IPv 6, Ethernet • The standard achieves high data rates in part via FEC and OFDM techniques • Has a long transmission range because – regulations allow high power transmissions – the use of directional antennas produces focused signals • Transmission range and data rate vary significantly depending on the frequency

The Physical Layer • Three types of physical layers are defined: – Wireless. MAN-SCa – Wireless. MAN-OFDM • A 256 -carrier OFDM. • Multiple access of different mobile terminals: TDMA – Wireless. MAN-OFDMA • A 2048 -carrier OFDM.

IEEE 802. 16 Extensions • IEEE 802. 16 a – used in licensed and ISM bands from 2 to 11 GHz. • At the lower ranges, the signals can penetrate barriers and thus do not require LOS between transmitter and receiver • IEEE 802. 16 b – increases the spectrum in 5 and 6 GHz frequency bands – provides Qo. S to ensure priority transmission

IEEE 802. 16 Extensions • IEEE 802. 16 c – represents a 10 to 66 GHz system profile • IEEE 802. 16 d (also called as Wi. Max) – includes minor improvements and fixes to 802. 16 a. – creates system profiles for compliance testing of 802. 16 a devices • IEEE 802. 16 e – standard for networking between fixed BSs and MSs. – would enable the high-speed signal handovers necessary for communications with users moving at vehicular speeds.

IEEE 802. l 6 d Wireless MAN-OFDM • Designed for NLOS operation • Operating frequencies: – 2 -11 GHz • Channel bandwidths: – 20 or 25 MHz (U. S. ) – 28 MHz (Europe)

Randomization • Randomization is performed on data transmitted on the downlink and uplink. • For each OFDM symbol randomizer shall be used independently • Pseudo Random Binary Sequence (PRBS) generator :

Randomization PRBS for data randomization

FEC • FEC: – Reed-Solomon, with variable block size and error correction capabilities – RS concatenated with punctured inner convolutional code – Turbo coding is optional – Space-time block codes are optional • Interleaving is also employed

FEC • Reed Solomon Code – Derived from a systematic RS (n=255, k=239 T=8) code using GF(m=8) – primitive polynomial specified as: [1 0 0 0 1 1 1 0 1] • Convolutional Code – Coding rate is ½ – Constraint length is 7 – Generator polynomials: • G 1=171 oct [1111001] • G 2=133 oct [1011011]

Convolutional Encoder

Puncturing • Puncturing is applied after CC • 1 means a transmitted bit 0 means a removed bit

Channel coding schematic • Modulation schemes: BPSK, QPSK, 16 -QAM and 64 -QAM • Pulse shaping: Square-root raised-cosine with a rolloff factor of 0. 25

Interleaving • Interleaving ensures that adjacent coded bits are mapped onto nonadjacent subcarriers • Block Interleaver with a two step permutations: – mk= (Ncbps/12). k mod 12+floor(k/12) k=0, 1. . Ncbps-1 – jk= s. floor(mk/s)+(mk+Ncbps-floor(12. mk/Ncbps)) mod(s) • De-interleaver is also defined by two step permutations – m =s. floor(j/s)+(j+floor(12. j/Ncpbs)) mod(s)

OFDM • OFDM symbol is made up from 256 subcarriers: – 192 data subcarriers – 8 pilot subcarriers – 56 null subcarriers: 28 lower, 27 higher frequency subcarriers for guard bands and one DC subcarrier • Pilot subcarriers are used to aid the receiver with synchronization and channel estimation. • Designated ratios of cyclic prefix time to useful time are 1/4, 1/8, 1/16, 1/32

Simulations

AWGN Channel - QPSK

AWGN Channel - 16 QAM

Rayleigh Fading Channel QPSK

Comparison • As we did not use pilot carriers for channel estimation, the use of the OFDM has harmful effects to the system varying with coherence time

Rayleigh Fading Channel – 16 QAM

Results and Conclusions • If we use OFDM without channel estimation we are encounter with serious problems in Rayleigh fading channels • Increasing the coherence time makes this worse because of the burst errors • It is obvious that QAM is useless in a Rayleigh fading because of its dependence on amplitude • Our next step will be channel estimation